1. Field
Embodiments of the present invention relate generally to an apparatus which converts an interlaced-scan video signal to a progressive-scan video signal, and more particularly to an apparatus which detects a subtitle in an interlaced-scan video signal such as a moving picture signal that is generated by 2-3 pull-down or 2-2 pull-down.
2. Description of the Related Art
In the case of an NTSC video signal that is generally used as a TV broadcast signal, a video image is scanned by an interlaced-scan system. In this system, the equivalent number of images per second is increased, and thereby surface flicker is reduced. On the other hand, in recent years, thin TV receivers, such as liquid crystal display (LCD) devices or plasma display devices, have steadily been gaining popularity. If interlaced-scan video signals are directly displayed on the scan lines on such thin TV receivers, the brightness of the screen would considerably lower and displayed images would not be worth viewing. To avoid this problem, in these display devices, images are displayed by a progressive-scan system. When an interlaced-scan video signal is to be displayed on a progressive-scan display device such as an LCD device, an interlaced-scan/progressive-scan conversion circuit is required.
In some cases, standard TV video signals, such as NTSC signals, include video signals which are produced on the basis of cinema films. The cinema film has 24 frames per second, while the standard TV video signal is an interfaced-scan video signal with 30 frames per second (60 fields per second). Thus, the video signal of the cinema film is converted to the standard TV video signal by a 2-3 pull-down method or a 2-2 pull-down method. In the description below, an interlaced-scan video signal, which is obtained by the conversion from the video signal of the cinema film, is referred to as “pull-down signal”.
For example, in the 2-3 pull-down method, a cinema film is first scanned to produce an interlaced-scan video signal with a frame frequency of 24 Hz. Then, the interlaced-scan video signal is converted such that the first frame of the cinema film may correspond to first and second fields (2 fields), the second frame to third to fifth fields (3 fields), the third frame to sixth and seventh fields (2 fields), and the fourth frame to eighth to tenth fields (3 fields). In the field signal which is converted to the pull-down signal, odd fields and even fields are alternately repeated. When the frame of the cinema film is converted to 3 fields, the last field (fifth field, tenth field) is the repetition of the same video content as the first field (third field, eighth field).
In this manner, two frames of the cinema film are made to correspond to five fields of the standard TV video signal, and the 2-field video signals and 3-field video signals are alternately repeated in accordance with the frames of the cinema film. Thus, the pull-down signal is generated.
In order to convert the pull-down signal to a progressive-scan signal, it is necessary to combine two fields, which are generated from the same frame of the cinema film, into one frame. A progressive-scan conversion apparatus, which realizes this process, determines whether the input video signal is a pull-down signal or not. If the input video signal is a pull-down signal, the apparatus determines fields to be combined (hereinafter referred to as “pair field”). By combining these fields, a progressive-scan signal is generated. This signal process is called “pull-down interpolation process”.
In some cases, video such as cinema includes a subtitle. Regardless of whether the video signal is a pull-down signal or not, the subtitle is configured such that the same character pattern is mixed in the video over a plurality of frames. In the case where a subtitle is included in the pull-down signal, it is difficult to determine whether the video signal is the pull-down signal or not.
Jpn. Pat. Appln. KOKAI Publication No. 2002-057993 (Patent Document 1) discloses a technique wherein when an input video signal is the above-described pull-down signal, a 2-3 pull-down interpolation process is executed if no subtitle region is detected, and an IP conversion signal is generated on the basis of a motion-adaptive interpolation process if a subtitle region is detected.
The above-mentioned Patent Document 1 does not describe a specific structure for detecting the subtitle. Thus, in the prior art, it is not possible to precisely detect a subtitle region and to exactly execute a pull-down interpolation process.